Make empty string behavior consistent on the legal search endpoint

[cnlucas]

What we’re after

We recently resolved a bug that caused certain filters in Swagger to return zero results when -- was selected. Now, selecting -- correctly returns all results for newer filters. While this fix has been applied to some filters, it should be consistently implemented across all filters.

We should also include this fix in the validation of the args as well.

It was first changed in the disposition ticket but was also updated in the subject ticket later. There's some differences between these two solutions, but the same outcome. (Adding '' to the validation and adding if empty string to the if statement before applying the query.

Related ticket(s)

(Include the tickets that either came before, after, or are happening in tandem with this new ticket)

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Action item(s)

(These are the smaller tasks that should happen in order to complete this work)

• All filters return all results when an empty string is queried

Completion criteria

(What does the end state look like - as long as this task(s) is done, this work is complete)

• [ ]

References/resources/technical considerations

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